1. Field of the Invention
The invention relates to a control device for an internal combustion engine.
2. Description of Related Art
Conventionally, an internal combustion engine in which an upstream-side exhaust gas control catalyst is provided in an exhaust passage of the internal combustion engine and a downstream-side exhaust gas control catalyst is provided in the exhaust passage on a downstream side of this upstream-side exhaust gas control catalyst in an exhaust gas flowing direction has widely been known (for example, Japanese Patent Application Publication No. 2011-069337 (JP 2011-069337 A)). Generally, an exhaust gas control catalyst that has an oxygen storage capacity is used in such an internal combustion engine.
When an oxygen storage amount is an appropriate amount between a maximum oxygen storable amount (an upper limit storage amount) and zero (a lower limit storage amount), the exhaust gas control catalyst that has the oxygen storage capacity can purify unburned gas (HC, CO, and the like), NOx, and the like in exhaust gas that flows into the exhaust gas control catalyst. In other words, when the exhaust gas at an air-fuel ratio that is richer than the theoretical air-fuel ratio (hereinafter also referred to as a “rich air-fuel ratio”) flows into the exhaust gas control catalyst, the unburned gas in the exhaust gas is oxidized and purified by oxygen that is stored in the exhaust gas control catalyst. On the contrary, when the exhaust gas at an air-fuel ratio that is leaner than the theoretical air-fuel ratio (hereinafter also referred to as a “lean air-fuel ratio”) flows into the exhaust gas control catalyst, oxygen in the exhaust gas is stored in the exhaust gas control catalyst. In this way, a surface of the exhaust gas control catalyst is brought into an oxygen deficient condition, and, in conjunction with this, NOx in the exhaust gas is reduced and purified. As a result, as long as the oxygen storage amount is the appropriate amount, the exhaust gas control catalyst can purify the exhaust gas regardless of the air-fuel ratio of the exhaust gas that flows into the exhaust gas control catalyst.
In view of the above, in a control device for such an internal combustion engine, in order to maintain the oxygen storage amount in the exhaust gas control catalyst at the appropriate amount, an air-fuel ratio sensor is provided on an upstream side of the upstream-side exhaust gas control catalyst in the exhaust gas flowing direction, and an oxygen sensor is provided on the downstream side thereof in the exhaust gas flowing direction. The control device uses these sensors to execute the feedback control on the basis of output of the air-fuel ratio sensor on the upstream side such that output of this air-fuel ratio sensor becomes a target value that corresponds to a target air-fuel ratio. In addition, the control device corrects the target value of the air-fuel ratio sensor on the upstream side on the basis of output of the oxygen sensor on the downstream side.
For example, in the control device described in JP 2011-069337 A, when output voltage of the oxygen sensor on the downstream side is a high-side threshold or larger and a state of the exhaust gas control catalyst is the oxygen deficient condition, the target air-fuel ratio of the exhaust gas that flows into the exhaust gas control catalyst is set to the lean air-fuel ratio. On the contrary, when the output voltage of the oxygen sensor on the downstream side is a low-side threshold or smaller and the state of the exhaust gas control catalyst is an oxygen excess condition, the target air-fuel ratio is set to the rich air-fuel ratio. According to JP 2011-069337 A, when the state of the exhaust gas control catalyst is the oxygen deficient condition or the oxygen excess condition, it is possible with such a configuration to promptly return the state of the exhaust gas control catalyst to an intermediate state between these states (that is, a state that an the appropriate amount of oxygen is stored in the exhaust gas control catalyst).
Meanwhile, in many internal combustion engines mounted in vehicles, fuel cut control for stopping a supply of fuel to a combustion chamber of the internal combustion engine is executed even during operation of the internal combustion engine. When such fuel cut control is executed, a large amount of oxygen flows into the upstream-side exhaust gas control catalyst and the downstream-side exhaust gas control catalyst.
Accordingly, when the fuel cut control is terminated, the oxygen storage amount of the upstream-side exhaust gas control catalyst reaches the maximum oxygen storable amount. Thus, the upstream-side exhaust gas control catalyst cannot store oxygen any more. For this reason, it has been suggested that, after the fuel cut control is terminated, the air-fuel ratio of the exhaust gas that flows into the upstream-side exhaust gas control catalyst is set to the rich air-fuel ratio so that oxygen is released from the upstream-side exhaust gas control catalyst (for example, Japanese Patent Application Publication No. 2009-036117 (JP 2009-036117 A), U.S. Pat. No. 6,226,982, Japanese Patent Application Publication No. 2013-024131 (JP 2013-024131 A), Japanese Patent Application Publication No. 2006-194118 (JP 2006-194118 A), and the like).